Figure 3-13. JFR Start Flight Recording window
Flight recordings are made in one of two modes: either for a fixed duration (1 minute in this
case), or continuously. For continuous recordings, a circular buffer is utilized; the buffer will
contain the most recent events that are within the desired duration and size.
To perform proactive analysis—meaning that you will start a recording and then generate
some work, or start a recording during a load-testing experiment after the JVM has warmed
up—a fixed-duration recording should be used. That recording will give a good indication of
how the JVM responded during the test.
The continuous recording is best for reactive analysis. This lets the JVM keep the most re-
cent events, and then dump out a recording in response to some event. For example, the We-
bLogic application server can trigger that a recording be dumped out in response to an abnor-
mal event in the application server (such as a request that takes more than 5 minutes to pro-
cess). You can set up your own monitoring tools to dump out the recording in response to
any sort of event.